Tensioners are used to apply a preload to belts used in engine accessory drives. A tensioner will typically comprise a pivot arm mounted to a base. A spring between the pivot arm and base urges the pivot arm, and a pulley journalled thereto, into engagement with a belt. This will apply a preload to the belt which assures proper power transmission from the belt to a driven accessory such as an alternator.
Belt alternator starter (BAS) systems for hybrid vehicles differ from conventional accessory drive belt systems in that the torque to turn the system is not always generated from the crankshaft. During normal operation the engine crankshaft provides torque for rotating a motor generator (MG) unit, or starter generator, generally providing taut and slack sides of the belt on opposite sides of the MG pulley. During engine starting, the MG unit drives the system causing the slack side of the belt to switch to the opposite side of the MG unit. Accordingly, two belt tensioners are generally required in such systems, engaging the belt on opposite sides of the MG pulley.
During normal engine driven operation of the generator and other accessories, the loads placed on the belt drive are determined by the power required to drive the accessories, including the MG unit. These accessory drive loads are relatively light and accordingly require belt tensioning only at a moderate low level. Contrariwise, during engine starting, the motor generator unit is required to provide power to rotate the engine as well as the connected accessories. This requires a higher level of belt tensioning to control motion on the slack side of the belt and assure that the belt will not slip. But the higher belt tension may put excessive tensioning loads on the belt and bearings of the system components during normal engine operation, which occupies the vast majority of operating time of the belt drive system. Also, because of packaging requirements it is usually necessary to provide separate tensioner arms with different pivot locations on each side of the MG unit to attain the necessary tensioner arm geometry. An improved accessory drive system, and belt tensioner are accordingly desired.
Representative of the art is U.S. Pat. No. 7,494,434 which discloses an accessory drive for an engine has a belt driven starter generator adapted for driving and being driven by the engine. In an exemplary embodiment, the drive includes a first engine drive pulley and a second starter drive pulley. A drive belt engages the drive pulleys for driving either pulley from the other. A dual belt tensioner made as a preassembled unit has a carrier with a central pivot mounted to the engine and first and second carrier arms extending radially from the central pivot. A first tensioner mounted on the first arm carries a first tensioner pulley biased against a first belt run adjacent the second drive pulley that is slack during engine starting. A second tensioner pulley carried on the second arm is biased against a second belt run adjacent the second drive pulley that is taut during engine starting A hydraulic strut connected to the second arm, and preferably included in the preassembled unit, provides moderate biasing for the second tensioner pulley during normal engine operation and velocity sensitive resistance, to increased belt forces, that limits reactive movement of the second tensioner pulley during engine starting and transient engine operation.
What is needed is a dual arm tensioner having a first pivot arm having a first pulley journalled thereto and engagable with a mounting surface through a first pivot axis, a second pivot arm having a second pulley journalled thereto, the second pivot arm pivotally engaged to the first pivot arm through a second pivot axis, the second pivot axis disposed from the first pivot axis. The present invention meets this need.